I think "just saying" means,"I'm stating my opinion and I will not defend it or engage with criticism of it: just saying, not discussing, debating, analysing or anything else. My mind is made up, I'm telling you what I think, and that's that."

It seems to be a very American phenomenon. I wonder if it is connected with American attitudes towards free speech? Freedom of speech is an important part of the American psyche. By "just saying" something, do we imply that any criticism of our opinion is actually a criticism of the fact that we "just said" something? Does not every American read the phrase "just sayin'" and think "Well, of course they have every right"?

Just exercisin' constitutional rights.